The generation of a T lymphocyte ("T cell") immune response is a complex process involving cell-cell interactions (Springer et al., A. Rev. Immunol. 5:223-252 (1987)), particularly between T and B cells, and production of soluble immune mediators (cytokines or lymphokines) (Dinarello and Mier, New Engl. Jour. Med. 317:940-945 (1987)). This response is regulated by several T-cell surface receptors, including the T-cell receptor complex (Weiss et al., Ann. Rev. Immunol. 4:593-619 (1986)) and other "accessory" surface molecules (Springer et al., (1987) supra). Many of these accessory molecules are naturally occurring cell surface differentiation (CD) antigens defined by the reactivity of monoclonal antibodies on the surface of cells (McMichael, Ed., Leukocyte Typing III, Oxford Univ. Press, Oxford, N.Y. (1987)).
One such accessory molecule is the CD28 antigen, a homodimeric glycoprotein of the immunoglobulin superfamily (Aruffo and Seed, Proc. Natl. Acad. Sci. 84:8573-8577 (1987)) found on most mature human T cells (Damle et al., J. Immunol. 131:2296-2300 (1983)). Current evidence suggests that this molecule functions in an alternative T cell activation pathway distinct from that initiated by the T-cell receptor complex (June et al., Mol. Cell. Biol. 7:4472-4481 (1987)). Monoclonal antibodies (mAbs) reactive with CD28 antigen can augment T cell responses initiated by various polyclonal stimuli (reviewed by June et al., supra). These stimulatory effects may result from mAb-induced cytokine production (Thompson et al., Proc. Natl. Acad. Sci 86:1333-1337 (1989); Lindsten et al., Science 244:339-343 (1989)) as a consequence of increased mRNA stabilization (Lindsten et al., (1989), supra). Anti-CD28 mAbs can also have inhibitory effects, i.e., they can block autologous mixed lymphocyte reactions (Damle et al., Proc. Natl. Acad. Sci. 78:5096-6001 (1981)) and activation of antigen-specific T cell clones (Lesslauer et al., Eur. J. Immunol. 16:1289-1296 (1986)).
The in vivo function of CD28 antigen is not known, although its structure (Aruffo and Seed, (1987), supra) suggests that like other members of the immunoglobulin superfamily (Williams and Barclay, Ann. Rev. Immunol. 6:381-405 (1988), it might function as a receptor. CD28 antigen could conceivably function as a cytokine receptor, although this seems unlikely since it shares no homology with other lymphokine or cytokine receptors (Aruffo and Seed, (1987) supra).
Alternatively, CD28 might be a receptor which mediates cell-cell contact ("intercellular adhesion"). Antigen-independent intercellular interactions involving lymphocyte accessory molecules are essential for an immune response (Springer et al., (1987), supra). For example, binding of the T cell-associated protein, CD2, to its ligand LFA-3, a widely expressed glycoprotein (reviewed in Shaw and Shimuzu, Current Opinion in Immunology, Eds. Kindt and Long, 1:92-97 (1988)), is important for optimizing antigen-specific T cell activation (Moingeon et al., Nature 339:314 (1988)). Another important adhesion system involves binding of the LFA-1 glycoprotein found on lymphocytes, macrophages, and granulocytes (Springer et al., (1987), supra; Shaw and Shimuzu (1988), supra) to its ligands ICAM-1 (Makgoba et al., Nature 331:86-88 (1988)) and ICAM-2 (Staunton et al., Nature 339:61-64 (1989)). The T cell accessory molecules CD8 and CD4 strengthen T cell adhesion by interaction with MHC class I (Norment et al., Nature 336:79-81 (1988)) and class II (Doyle and Strominger, Nature 330:256-259 (1987)) molecules, respectively. "Homing receptors" are important for control of lymphocyte migration (Stoolman, Cell 56:907-910 (1989)). The VLA glycoproteins are integrins which appear to mediate lymphocyte functions requiring adhesion to extracellular matrix components (Hemler, Immunology Today 9:109-113 (1988)). The CD2/LFA-3, LFA-1/ICAM-1 and ICAM-2, and VLA adhesion systems are distributed on a wide variety of cell types (Springer et al., (1987), supra; Shaw and Shimuzu, (1988,) supra and Hemler, (1988), supra).
Intercellular adhesion interactions mediated by integrins are strong interactions that may mask other intercellular adhesion interactions. For example, interactions mediated by integrins require divalent cations (Kishimoto et al., Adv. Immunol. 46:149-182 (1989). These interactions may mask other intercellular adhesion interactions that are divalent cation independent. Therefore, it would be useful to develop assays that permit identification of non-integrin mediated ligand/receptor interactions.
T cell interactions with other cells such as B cells are essential to the immune response. Levels of many cohesive molecules found on T cells and B cells increase during an immune response (Springer et al., (1987), supra; Shaw and Shimuzu, (1988), supra; Hemler (1988), Supra). Increased levels of these molecules may help explain why activated B cells are more effective at stimulating antigen-specific T cell proliferation than are resting B cells (Kaiuchi et al., J. Immunol. 131:109-114 (1983); Kreiger et al., J. Immunol 135:2937-2945 (1985); McKenzie, J. Immunol. 141:2907-2911 (1988); and Hawrylowicz and Unanue, J. Immunol. 141:4083-4088 (1988)). The fact that anti-CD28 mAbs inhibit mixed lymphocyte reactions (MLR) may suggest that the CD28 antigen is also an adhesion molecule.
Optimal activation of B lymphocytes and their subsequent differentiation into immunoglobulin secreting cells is dependent on the helper effects of major histocompatibility complex (MHC) class II antigen (Ag)-reactive CD4 positive T helper (CD4.sup.+ T.sub.h) cells and is mediated via both direct (cognate) T.sub.h -B cell intercellular contact-mediated interactions and the elaboration of antigen-nonspecific cytokines (non-cognate activation; see, e.g. Noel and Snow, Immunol. Today 11:361 (1990)). Although T.sub.h -derived cytokines can stimulate B cells (Moller, Immunol. Rev. 99:1 (1987)), their synthesis and directional exocytosis is initiated and sustained via cognate interactions between antigen-primed T.sub.h cells and antigenpresenting B cells (Moller, Supra). The successful outcome of T.sub.h -B interactions requires participation of transmembrane receptor-ligand pairs of co-stimulatory accessory/adhesion molecules on the surface of T.sub.h and B cells which include CD2 (LFA-2); CD58 (LFA-3), CD4:MHC class II, CD11a/CD18 (LFA-1):CD54 (1CAM-1).
During cognate T.sub.h :B interaction, although both T.sub.h and B cells cross-stimulate each other, their functional differentiation is critically dependent on the provision by T.sub.h cells of growth and differentiation-inducing cytokines such as IL-2, IL-4 and IL-6 (Noel, supra, Kupfer et al., supra, Brian, supra and Moller, supra). Studies by Poo et al. (Nature 332:378 (1988)) on cloned T.sub.h :B interaction indicate that interaction of the T cell receptor complex (TcR) with nominal Ag-MHC class II on B cells results in focused release of T.sub.h cell-derived cytokines in the area of T.sub.h and B cell contact (vectorially oriented exocytosis). This may ensure the activation of only B cells presenting antigen to T.sub.h cells, and also avoids activation of bystander B cells.
It was proposed many years ago that B lymphocyte activation requires two signals (Bretscher and Cohn, Science 169:1042-1049 (1970)) and now it is believed that all lymphocytes require two signals for their optimal activation, an antigen specific or clonal signal, as well as a second, antigen non-specific signal (Janeway, supra). The signals required for a T helper cell (T.sub.h) antigenic response are provided by antigen-presenting cells (APC). The first signal is initiated by interaction of the T cell receptor complex (Weiss, J. Clin. Invest. 86:1015 (1990)) with antigen presented in the context of class II major histocompatibility complex (MHC) molecules on the APC (Allen, Immunol. Today 8:270 (1987)). This antigen-specific signal is not sufficient to generate a full response, and in the absence of a second signal may actually lead to clonal inactivation or anergy (Schwartz, Science 248:1349 (1990)). The requirement for a second "costimulatory" signal provided by the MHC has been demonstrated in a number of experimental systems (Schwartz, supra; Weaver and Unanue, Immunol. Today 11:49 (1990)). The molecular nature of these second signal(s) is not completely understood, although it is clear in some cases that both soluble molecules such as interleukin (IL)-1(Weaver and Unanue, Supra) and membrane receptors involved in intercellular adhesion (Springer, Nature 346:425 (1990)) can provide costimulatory signals.
Freeman et al. (J. Immunol. 143(8):2714-2722 (1989)) isolated and sequenced a cDNA clone encoding a B cell activation antigen recognized by mAb B7 (Freeman et al., J. Immunol. 138:3260 (1987)). COS cells transfected with this cDNA have been shown to stain by both labeled mAb B7 and mAb BB-1 (Clark et al., Human Immunol. 16:100-113 (1986); Yokochi et al., J. Immunol. 128:823 (1981)); Freeman et al., (1989) supra; and Freedman et al., (1987), supra)). Expression of the B cell activation antigen has been detected on cells of other lineages. For example, studies by Freeman et al. (1989) have shown that monocytes express low levels of mRNA for B7.
Expression of soluble derivatives of cell-surface glycoproteins in the immunoglobulin gene superfamily has been achieved for CD4, the receptor for HIV-1, using hybrid fusion molecules consisting of DNA sequences encoding portions of the extracellular domain of CD4 receptor fused to antibody domains (human immunoglobulin C gamma 1), as described by Capon et al., Nature 337:525-531 (1989).
While the CD28 antigen has functional and structural characteristics of a receptor, until now, a natural ligand for this molecule has not been identified. It would be useful to identify ligands that bind with the CD28 antigen and other receptors and to use such ligand(s) to regulate cellular responses, such as T cell and B cell interactions, for use in treating pathological conditions.